The present invention relates to a light guide plate and a surface illumination device. Specifically, the present invention relates to a surface illumination device that is used as a backlight for a liquid crystal display device and a light guide plate that is used in the surface illumination device.
Nowadays, there is an increasing demand for a thin surface illumination device with progress of a thinner mobile device in which the surface illumination device is incorporated. In order to thin the surface illumination device, it is necessary to decrease a thickness of a light guide plate. However, there is a limitation to a decrease in height of a light source constructed by LEDs even if the thickness of the plate-like light guide plate can be decreased. Therefore, in the case that the plate-like thin light guide plate is used, the height of the light source is greater than the thickness of an end face (a light incident surface) of the light guide plate, and the light source, which is disposed while opposed to the light incident surface of the light guide plate, projects upward from an upper surface of the light guide plate. When the light source projects upward from the light guide plate, light emitted from the light source is not completely incident to the light incident surface of the light guide plate, but the light leaks partially to the outside to degrade light use efficiency.
Use of a light guide plate is proposed in order to solve the trouble. In the light guide plate, a light introduction unit in which the thickness is greater than that of a plate-like light guide plate body is provided at an end of the light guide plate body, and an inclined surface that is inclined toward the end of the light guide plate body from a point of the maximum thickness of the light introduction unit is provided in the light introduction unit. For example, International Publication No. WO2010/070821 and International Publication No. WO 2008/153024 disclose a surface illumination device in which the light guide plate is used.
FIG. 1 illustrates an example of a surface illumination device 11 in which the light guide plate including the light introduction unit having the thickness greater than that of the light guide plate body is used. A light guide plate 13 includes a light guide plate body 14 having the substantially even thickness and a light introduction unit 15 formed into a wedge shape. A deflection pattern or a diffusion pattern is formed in a rear surface of the light guide plate body 14, and a lenticular lens 16 is formed in a surface. An inclined surface 17 that is inclined toward the end of the light guide plate body 14 from the point of the maximum thickness of the light introduction unit 15 is provided in the light introduction unit 15. The thickness of the end face (the light incident surface) of the light introduction unit 15 is greater than the height of the light source 12. In the surface illumination device 11 in which the light guide plate 13 is used, the thickness of the end face of the light introduction unit 15 is greater than the height of the light source 12, whereby the light emitted from a light source 12 is efficiently taken in the light introduction unit 15. The light taken in the light introduction unit 15 is guided to the light guide plate body 14 to spread into a planar shape, and the light is deflected by the deflection pattern or the diffusion pattern to exit to the outside from the light exit surface of the light guide plate body 14. At this point, a directional pattern of the light exiting from the light exit surface is spread by the lenticular lens 16. Accordingly, a balance between improvement of the light use efficiency of the light source and the thinner surface illumination device can be established in the surface illumination device having the above structure.
However, in the light emitted from the light source (the LED light source), color changes gradually as a ray direction is inclined from a front direction (hereinafter referred to as a direction of an optical axis C) of the light source. Therefore, in the surface illumination device, unfortunately color unevenness is generated between the light exiting from the light exit surface in front of the light source and the light exiting from the light exit surface in a side end portion of the light guide plate.
FIGS. 2A and 2B illustrate a change in color (an average chromaticity value along a straight line of a direction φ) of the light in the direction φ when the light is emitted in the direction φ based on the direction of the optical axis C of the light source 12 as illustrated in FIG. 2C. In FIGS. 2A and 2B, chromatic display is illustrated using a CIE xy chromaticity diagram (for example, see “CIE's XYZ Coordinate System”: http://fourier.eng.hmc.edu/e180/lectures/color1/node25.html) defined by International Commission on Illumination (CIE). In the CIE xy chromaticity diagram, a specific color can be expressed or designated by a pair of an x-coordinate and a y-coordinate. In FIGS. 2A and 2B, a horizontal axis expresses a light exit direction φ that is measured based on the direction of the optical axis C. In FIG. 2A, a vertical axis expresses a difference Δx=x1−x0 between a value x1 of the x-coordinate of the chromaticity diagram indicating the color of the light exiting in the direction φ and a value x0 of the x-coordinate of the chromaticity diagram indicating the color of the light exiting in the direction of the optical axis C (φ=0°). Similarly, in FIG. 2B, the vertical axis expresses a difference Δy=y1−y0 between a value y1 of the y-coordinate of the chromaticity diagram indicating the color of the light exiting in the direction φ and a value y0 of the y-coordinate of the chromaticity diagram indicating the color of the light exiting in the direction of the optical axis C (φ=0°).
As illustrated in FIGS. 2A and 2B, the color of the light emitted from the light source changes depending on the light exit direction φ. For this reason, the light incident to the light guide plate is colored in a different color according to the light guide direction, and possibly the color unevenness is generated in the light exit surface. For the surface illumination device 11 in which the light introduction unit 15 includes the inclined surface 17 as illustrated in FIG. 1, because the light guided toward the optical axis direction of the light source 12 in the light guide plate 13 and the light guided toward the direction inclined from the optical axis direction do not sufficiently mix with each other, the color unevenness is generated between a central portion (an A portion in FIG. 1) and both side end portions (B portions in FIG. 1) of the light guide plate.
FIG. 3 is a perspective view of the surface illumination device disclosed in International Publication No. WO2010/070821. In a surface illumination device 21 in FIG. 3, a light leak prevention pattern 22 including plural V-grooves parallel to each other is provided in the inclined surface 17 of the light introduction unit 15. The light leak prevention pattern 22 decreases the light leakage from the inclined surface 17 to improve the light use efficiency.
In the surface illumination device 21, because the light guide direction of the light reflected by the light leak prevention pattern 22 is bent into a width direction of the light guide plate, it is expected that the generation of the color unevenness is reduced by the mixture of the pieces of light having the different colors. However, actually the reduction of the color unevenness is restrictive and insufficient even in this structure.
In view of the problems described above, it is desired to provide a surface illumination device and a light guide plate for being able to reduce the generation of the color unevenness between the light source front region and both the side end portions of the light guide plate body in the light exit surface.